Clinical trials of live attenuated Shigella strains administered orally at the CVD protected only a minority of vaccinees from shigellosis upon challenge with wild-type. In a follow up study, these volunteers were re-challenged. Most of these who had been exposed to wild-type Shigella were protected from the symptoms of the disease. This study confirms field evidence in suggesting that protection can be conferred by exposure to Shigella. The presence of antibodies to Shigella measured in these volunteers did not correlate with resistance to illness following challenge. It is well established that cell-mediated immunity is the key to protection against many intracellular pathogens, including bacteria. Recent studies have suggested that cell mediated immunity may play a significant role in combating shigellosis. The goal of this research is to assess whether systemic cell mediated immunity correlates with protection from shigellosis. If such a parameter is found it will considerably enhance our understanding of the mechanisms underlying protection in shigellosis and the development of Shigella vaccines. Specifically, using peripheral blood mononuclear cells (PBMC) obtained from volunteers immunized with both wild-type and attenuated strains of S. flexneri 2a, we propose to: (1) Test the hypothesis that inoculation of volunteers with attenuated strains or wild-type S.flexneri 2a elicits the appearance in circulation of specific cytotoxic T lymphocyte responses (CTL). If CTL activity is indeed observed, we will test the hypothesis that this immune response is mediated by CD8+ cells and that it is restricted by class I MHC molecules. (2) Test the hypothesis that, in addition to CTL responses, inoculation of volunteers with attenuated strains or wild-type S.flexneri 2a elicits the appearance in circulation of specific T lymphocytes that proliferate and produce interferon-gamma (and type-1" cytokines) in response to Shigella antigens. If proliferative and/or cytokine responses are observed, we will test the hypothesis that these immune response are mediated by CD4+ cells and are restricted by class II MHC molecules. (3) Test the hypothesis that CTL responses and/or interferon-gamma production correlates with protection to challenge with virulent Shigella strains by correlating these immune effector mechanisms with protection in volunteers challenged with wild-type S.flexneri 2a. (4) Determine whether protective epitopes can be identified in Shigella antigens by studying the fine antigen specificity of T cell clones derived from PBMC obtained from volunteers that were protected or non-protected after challenge with wild-type S.flexneri 2a.